1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to an electrode of an electron gun for forming a large-diameter electronic lens and an electron gun using the same.
2. Description of the Related Art
In general, spherical aberration and focusing characteristics in an electron gun for a CRT, are greatly affected by a main lens. Thus, in order to obtain good focusing characteristics, it is preferable to form a main lens having a diameter as large as possible.
However, in an in-line type electron gun, each three electron beam passing holes are formed in an in-line arrangement in at least two electrodes for forming an electron lens, and the diameter of a neck portion of a funnel in which the electron gun is mounted is limited. Thus, it is not possible to make the diameter of an electron beam passing hole larger than a distance between centers of two neighboring electron beam passing holes, which will be referred to as an xe2x80x9ceccentricity distancexe2x80x9d hereinafter.
Electrodes of an electron gun for improving spherical aberration in a conventional main lens are disclosed in U.S. Pat. No. 4,370,592, which is shown in FIG. 1.
As shown in the drawing, burring portions 1b and 2b are formed at edges of an emitting surface 1a of a focusing electrode 1 and an entering surface 2a of a final accelerating electrode 2, and large-diameter electron beam passing holes 1H and 2H having a predetermined depth, are formed in the central portion thereof, respectively. Also, small-diameter electron beam passing holes 1Hxe2x80x2 and 2Hxe2x80x2 through which R, G and B electron beams pass independently are formed in the large-diameter electron beam passing holes 1H and 2H.
When electron beams pass through a main lens formed by the focusing electrode 1 and the final accelerating electrode 2, since the large-diameter electron beam passing holes 1H and 2H are horizontally elongated, the vertically and horizontally focused components of the electron beams having passed through the central small diameter electron beam passing hole and the side small-diameter electron beam passing holes are different. Thus, it is not possible to obtain uniformity in the spot size of electron beams landing on a phosphor screen. In other words, as shown in FIG. 2, the side electron beams RB and BB having passed through the large-diameter electron beam passing hole 1H or 2H of the focusing electrode 1 or the final accelerating electrode 2 are close to the burring portions 1b and 2b, and the central electron beams GB is relatively far from the burring portions 1b and 2b. Therefore, the side electron beams RB and BB are relatively strongly focused and the central electron beam GB is relatively weakly focused.
Also, since the distances between the side electron beams RB and BB and the burring portions 1b and 2b are different according to direction, that is, horizontally or vertically, horizontal and vertical focusing powers of the side electron beams RB and BB are different. Also, since the vertical distances between the central electron beam GB and the burring portions 1b and 2b are shorter than the horizontal distances therebetween, the central electron beam GB is strongly focused in a vertical direction. Also, the central electron beam GB is diverged in a diagonal direction of the large-diameter electron beam passing hole 1H or 2H. Therefore, the side electron beams RB and BB having passed through the main lens have substantially triangular cross-sections and the central electron beam GB has a cross-section having radially projecting parts, so that a uniform shape in electron beam cross-sections cannot be obtained throughout the entire phosphor screen.
In particular, since the sizes of the small-diameter electron beam passing holes 1Hxe2x80x2 and 2Hxe2x80x2 are restricted by the diameter of the neck portion of a CRT, there is a limit in increasing the eccentricity distance between the small-diameter electron beam passing holes 1Hxe2x80x2 and 2Hxe2x80x2. Further, in recent years, there has been a tendency to reduce the diameter of the neck portion for reducing a deflection current, the distance between the small-diameter electron beam passing holes 1Hxe2x80x2 and 2Hxe2x80x2 is reduced accordingly, thereby degrading spherical aberration and focusing characteristics.
An electrode structure of an electron gun for solving the above-mentioned problem is disclosed in U.S. Pat. No. 5,414,323. As shown in FIG. 3, the electrode structure is constructed such that an electrode member 12 is disposed at the center of an outer electrode 11 having a large-diameter electron beam passing hole, a vertically elongated small-diameter electron beam passing hole 13 is formed in the center of the electrode member 12 and both edges of the electrode member 12 are recessed in a half-elliptical shape to form side electron beam passing holes 14 and 15.
The central small-diameter electron beam passing hole 13 is vertically elongated to offset astigmatism generated by the large-diameter electron beam passing hole. However, this electrode structure cannot easily correct 8-pole coma aberration of a central electron beam passing hole and 6-pole coma aberration of side electron beam passing holes.
An example of another conventional large-diameter electrode is disclosed in U.S. Pat. No. 4,626,783. This electrode, as shown in FIG. 4, includes an outer electrode 21 having a large-diameter electron beam passing hole, and an inner electrode 22 installed within the outer electrode 21 and having polygonal small-diameter electron beam passing holes 22R, 22G and 22B. Here, the aberration generated by the large-diameter electron beam passing hole can be corrected by the polygonal small-diameter electron beam passing holes 22R, 22G and 22B. However, it is not easy to fabricate polygonal small-diameter electron beam passing holes.
To solve the above problems, it is an object of the present invention to provide an electrode of an electron gun for a color cathode ray tube, which can easily correct aberration of an electronic lens caused by a large-diameter electron beam passing hole and can improve focusing characteristics.
It is another object of the present invention to provide an electron gun for a color cathode ray tube, which can reduce astigmatism by compensating for distortion of an electron beam due to a difference in the voltage applied to three electron beam passing holes disposed in an in-line arrangement.
To accomplish the first object of the present invention, there is provided an electrode of an electron gun for a color cathode ray tube including an outer-rim electrode having a large-diameter electron beam passing hole through which three electron beams pass, and an inner electrode installed inside the outer-rim electrode member, and having three electron beam passing holes disposed in an in-line arrangement and recesses formed at peripheries of the electron beam passing holes, the recesses having an eccentricity distance larger than an eccentricity distance between centers of the three electron beam passing holes.
In the present invention, the horizontal width of each of the recesses formed at peripheries of the electron beam passing holes is preferably smaller than the vertical width thereof.
According to another aspect of the present invention, there is provided an electron gun for a cathode ray tube, the electron gun having a cathode, a control electrode and a screen electrode together constituting a triode section, and focusing electrodes installed to be adjacent to the screen electrode and forming at least one electronic lens, wherein each of the focusing electrodes includes an outer-rim electrode having a large-diameter electron beam passing hole through which three electron beams pass, and an inner electrode installed inside the outer-rim electrode member, and having three electron beam passing holes disposed in an in-line arrangement and recesses formed at peripheries of the electron beam passing holes, the recesses having an eccentricity distance larger than an eccentricity distance between centers of the three electron beam passing holes.
Preferably, the horizontal widths of the large-diameter electron beam passing holes formed in the outer-rim electrodes are different.